It is known in the state of the art for the use of LED array driver devices in applications like displays, information and advertising panels, traffic signals, automotive lighting.
One of the most important parameters for these devices is the current accuracy among all the channels of the array; the parameter that mainly affects this accuracy is the OFFSET of the OPAMP used in the control loop of the drivers.
FIG. 1 shows a prior art current driver for a LED diode wherein a current generator Iled flows through a resistance Rref connected to ground GND. An operational amplifier OP1 drives by means of its output the gate terminal of a MOS transistor T1 having the source terminals coupled to ground GND by means of a resistance Rsense and the drain terminal connected to the output pad PAD; the output pad Pad is connected to at least one LED diode LED1 wherein a current Iled1 flows. The non-inverting input terminal of the operational amplifier OP1 is connected to the common terminal of the current generator Iled and the resistance Rref, while the inverting terminal is connected to the source terminal of the MOS transistor T1.
The operational amplifier OP1 must make the voltage Vsense across the resistance Rsense equal to the voltage Vref across the resistance Rref. Of course in real circuits these voltages are not identical because of the offset voltage of the operational amplifier.
FIG. 2 shows the circuit in FIG. 1 but wherein the effect of the offset voltage is taken into account. Here a real operational amplifier is modeled with an ideal operational amplifier with a voltage source in one of its inputs, representing the offset voltage Voffset; more precisely the offset voltage Voffset is arranged at the inverting terminal of the operational amplifier OP1. In this case:
            Vsense      =              Vref        -        Voffset              ;        since    ⁢                      Iled    =          Vsense      Rsense      
and considering also that the voltage Voffset could have also the opposite polarity, in general:
      Iled    ⁢                  ⁢    1    =            Vref      Rsense        ±          Voffset      Rsense      
The accuracy of the current Iled1 increases by reducing the offset voltage.
Typical LED array driver devices have a channel to channel current accuracy better than 3%. By reducing the offset voltage from 5 mV to few microvolts, the estimated channel to channel accuracy will be better than 1%.